1. Field of the Invention
The present invention relates to a screw compressor for refrigerating apparatus that is driven by a motor being controlled by an inverter.
2. Description of the Related Art
Conventionally, a screw compressor for refrigerating apparatus adapted to control the rotational speed of each of motors for driving the compressor body and the liquid injection pump according to the increase and the decrease of load is known in Japanese Patent Laid-Open Publication No. Sho 57-18484. This screw compressor has a suction pressure detector for detecting the suction pressure in its suction section, which pressure has a monotone increasing relationship with the cooling heat load, wherein a pressure signal outputted from the suction pressure detector is inputted to a variable voltage frequency converter via a pressure regulator, so that rotational speeds of a compressor body driving motor and a liquid injection pump driving motor are controlled simultaneously.
In addition, a screw compressor for refrigerating apparatus which is aimed at the improvement of efficiency in a low partial load region by adding a capacity control system using an inverter to a stepless capacity control mechanism by a slide valve is disclosed in Japanese Patent Laid-Open Publication No. Sho 59-211790.
The above-mentioned screw compressor disclosed in Japanese Patent Laid-Open Publication No. Sho 57-18484 is controlled to increase the rotational speed of motor when the pressure detected by the suction pressure detector is higher than the predetermined value of pressure. Therefore, there is a problem in that the motor is apt to be overloaded, so that the endurance of the motor is deteriorated.
Furthermore, the screw compressor disclosed in Japanese Patent Laid-Open Publication No. Sho 59-211790 has a complicated construction because the slide valve is provided. Furthermore, the screw compressor is constructed so that the inverter is not used, i.e., a commercial power source is directly used without intervening the inverter, when the capacity of this screw compressor is in the range of 100 to 75% by the slide valve, whereby there is a problem in that the inverter is not sufficiently used.
In view of above problems, it is an object of the present invention to provide a screw compressor for refrigerating apparatus capable of controlling the rotational speed of a motor to be suitable for cooling heat load by an online power system using an inverter, without causing overload of the motor.
To solve the above problem, the present invention provides a screw compressor for refrigerating apparatus comprising: a screw rotor; a motor for driving said screw rotor, the rotational speed of said motor being controlled through an inverter; a heat load detecting means for detecting a cooling heat load; a load condition detecting means for detecting the heat load condition of said motor; and a motor rotational speed controlling device for controlling the rotational speed of said motor based on a heat load signal from said heat load detecting means and a load condition signal from said load condition detecting means.
Here, said motor rotational speed controlling device can be configured to control the rotational speed of said motor to be reduced if the compressor capacity is determined as excessive based on said heat load signal or if the load condition of said motor is determined as excessive based on said load condition signal, and to control the rotational speed of said motor to be increased if the compressor capacity is determined as lacking based on said heat load signal and the load condition of said motor is determined as not excessive based on said load condition signal.
As said head load detecting means, either a detector for detecting the suction pressure of said screw compressor or a detector for detecting the temperature of cooled liquid, emanated from an evaporator in said refrigerating apparatus can be used.
As said load condition detecting means, a detector for detecting the coil temperature of said motor, a detector for detecting the electric current that is supplied to said motor, a detector for detecting the temperature of refrigerant gas that is discharged from said screw compressor, a detector for detecting the rotational speed of said motor and the like can be used.
It is possible to provide a plurality of said load condition detecting means to perform the control of rotational speed of said motor based on a plurality of said load condition signals.
According to the present invention, it becomes possible to control the corresponding rotational speed of the motor to be suitable for the cooling heat load without causing the overload of motor. As a result, the improvement of motor endurance and the reduction of consuming power can be realized. Furthermore, above advantages are accomplished by always using the inverter and not using a slide valve.